In diode-laser pumped, digitally modulated, solid-state lasers and OPS lasers a predetermined output power level is set and analog-controlled automatically by monitoring power output of the laser, comparing that monitored power with a preset level, and adjusting optical pump power by adjusting the drive current of the diode-laser (or array thereof) to maintain the output power at the pre-set level. The laser is digitally modulated by switching the diode-laser current on and off with the “on” current value being that corresponding to the preset power level.
FIG. 1 schematically illustrates a typical arrangement 10 of such a diode-laser pumped, digitally modulated, solid-state laser. Here, laser-radiation paths are designated by fine lines, and electrical connections are designated by bold lines. Arrangement 10 includes a laser optics unit 12, including a laser resonator 14. Resonator 14 includes a solid-state gain medium (not shown) which is energized (pumped) by radiation RP from a diode-laser radiation source 16. Source 16 can be a single diode-laser or an array of such diode lasers.
In response to the energizing (pumping), laser resonator 14 delivers radiation RF having a fundamental wavelength characteristic of the gain medium to an optional frequency convertor 18. The frequency convertor can include one or more optically nonlinear crystals arranged to convert radiation RF to radiation RC having a wavelength different from the fundamental wavelength. By way of example, the frequency convertor can include one crystal arranged to convert the fundamental-wavelength radiation to second-harmonic (2H) radiation, or two crystals arranged to convert the fundamental-wavelength radiation to third-harmonic (3H) or fourth-harmonic (4H) radiation. Alternatively, frequency-conversion can be carried out by one or more crystals located within the laser resonator. In either case, the frequency-converted radiation provides the laser output-radiation.
A pick-off mirror 20 directs a sample 22, for example about 1%, of the output radiation to a photo-detector (photo-diode) 24 the output of which is connected to a detector calibration circuit 26. The detector calibration circuit 26 sends a signal 27 representative of the instant actual laser output power to light (output radiation) regulation circuitry 28. Light regulation circuitry 28 receives an input signal 29 representative of a desired laser output power. Based on a comparison of signals 27 and 29, light regulation circuitry 28 communicates a signal 30 to a diode-laser current-source 32 which varies the output current 38 of the current-source until the actual output power laser 12 matches the desired output power.
Modulation of the output of laser 12 is achieved by switching the output of the current-source between zero or some predetermined bias (minimum) current 36 and a maximum value determined instantaneously by the signal from the light regulator circuitry. Switching is accomplished by a digital modulation input signal 34 (going low-to-high or high-to-low) from an external source (not shown) such as a microprocessor or a PC. Bias current 36 provides for faster switching (modulation) of laser 12 from an “off” condition to an on condition at the expense of a lower modulation depth or contrast ratio of the laser output.
FIG. 2 is a functional circuit diagram schematically illustrating details of one example 40 of the light regulation and modulation circuitry of FIG. 1. Here detector calibration circuit 26 includes a variable resistor R1 connected in series with photo-diode 24 of FIG. 1. Signal 27 from the calibration circuitry is connected via a fixed resistor R2 to one input of an operational amplifier 42. The set power signal 29 is connected to the other input of the operational amplifier. Amplifier 42, here, is configured as an integrator, using the combination of R3 and C1 as a feedback loop, to optimize gain at low modulation frequencies. Output 30 of the operational amplifier is communicated to the diode-laser current source 32.
A principal disadvantage of the light-regulation and modulation method of FIGS. 1 and 2 is that the time required for the power output to stabilize at the set level after modulation “turn-on” is inversely dependent on the set level value. By way of example, a diode-laser pumped, external cavity surface-emitting semiconductor laser (OPS laser) having a fundamental wavelength of 976 nm intra-cavity frequency-doubled to provide output radiation having a wavelength 488 nm was tested to determine stabilization time as a function of set-power. Intra-cavity frequency doubling was achieved using a lithium borate (LBO) crystal. At a set output level of 20 milliwatts (mW) power stabilized at the set level at about 20 microseconds (μs) after turn-on. When the set level was reduced to 3 mW, about 200 μs were required for the output to stabilize at the set level.
A primary reason for this inverse dependence of stabilizing time on set-power level is that the gain (response time) of the light regulation circuit is limited by the laser build-up time, thereby reducing the maximum possible gain for higher modulation frequencies. The build-up time is essentially dead time for the light regulation circuit, limiting the rise time of the circuitry.
FIG. 3A, FIG. 3B, and FIG. 3C together form a timing diagram schematically illustrating the operation of the apparatus of FIGS. 1 and 2 for one cycle of digital modulation turned on at time t0 and off at time t1. Light regulation is in operation during the whole digital modulation cycle. It can be seen that for a low set-level of laser power current ramp-up and corresponding output power ramp-up to stabilized values take longer than for a higher set-level. This is because the gain of the light regulator amplifier is proportional to the difference between the actual power and the set power.
This, unfortunately, means that stabilized power is delivered for a shorter time during any fixed digital modulation cycle the lower the desired power level. This is unfortunate, because in a digitally modulated laser the modulation frequency is often required to be the same for both low peak power and high peak power.
There is a need for a method of operating a digitally modulated diode-laser pumped solid-state laser such that the delivery time for stabilized laser power is about the same for any desired output power of the laser, and stabilized power is delivered through most of the digital modulation cycle. Preferably, high modulation frequency should be possible, independent of the stabilized level of laser output power.